Ventricular arrhythmias are potentially lethal. In the instance of chaotic, non-coordinated cardiac contraction, known as fibrillation, death may occur within minutes. For patients who have survived an episode of ventricular fibrillation, there is a high probability of recurrence. In addition, patients who have experienced sustained symptomatic ventricular tachycardia are at risk in that such arrhythmias may convert to fibrillation. It is these patients who may benefit from an implantable cardioverter or defibrillator.
Many patients are afflicted with cardiac disease such that a potentially lethal ventricular arrhythmia is possible, but not sufficiently likely to warrant the trauma, inconvenience and costs associated with an implanted anti-arrhythmia device. These patients may be better served by an alarm device which warns of the occurrence of an arrhythmia but does not apply therapy to the heart. Thus, when the alarm detects an arrhythmia condition and emits a warning signal, the patient may seek medical assistance.
Such an alarm device may also benefit patients having an implanted cardioverter or defibrillator. In these patients, an implantable cardiac condition alarm provides an early warning alarm signal so that the patient may take precautionary steps to reduce the threat associated with sudden incapacity due to an arrhythmia condition. For example, when ventricular fibrillation occurs, a patient commonly reaches an unconscious state within five to fifteen seconds following the onset of fibrillation due to lack of oxygen to the brain. If such a patient experiences ventricular fibrillation while performing an activity, such as standing or driving an automobile, a warning signal may allow the patient to take an appropriate action to avoid the consequences of becoming suddenly unconscious. Thus, a patient is alerted to the onset of a cardiac abnormality to terminate a risky activity or to obtain medical attention when it becomes practical.
U.S. Pat. No. 4,295,474, entitled "Recorder with Patient Alarm and Service Request Systems Suitable for use with Automatic Implantable Defibrillator", issued Oct. 20, 1981, to R. E. Fischell, discloses a system and apparatus, particularly adapted for use with an automatic implanted defibrillator, that monitors electrocardiogram data signals to provide a continuously updated recording of the ECG data. The recorder responds to the operating conditions of the automatic implanted defibrillator device and places into electronic storage, for subsequent readout to the patient's doctor, ECG data both immediately preceding the onset of ventricular fibrillation and also during the subsequent defibrillation activity, when one or more high energy electrical impulses are applied to the patient's heart. In addition, this recorder operates to automatically alert the patient when ventricular fibrillation has been detected and defibrillation is to be attempted, so that appropriate precautions may be taken. Moreover, following defibrillation, the patient is alerted in a distinctive manner that defibrillation has occurred and that a physician should be contacted.
The Fischell monitoring device requires electrode leads for sensing ECG signals. In an implanted device, the leads are generally extended through the patient's body, through the circulatory system to make a direct electrical connection with the heart. Some trauma to the patient is involved in implanting the leads. Further disadvantages of the leads are that they are subject to breakage and the point of connection between the leads and the monitor may allow leakage of biological fluids into the monitor.
U.S. Pat. No. 5,113,869, entitled "Implantable Ambulatory Electrocardiogram Monitor", which issued to T. A. Nappholz et al. on May 19, 1992 describes an implanted programmable ambulatory electrocardiography (AECG) patient monitoring device that chronically senses and analyzes electrocardiographic signals from at least one subcutaneous precordial sensor to detect electrocardiogram and physiological signal characteristics predictive of cardiac arrhythmias. The device includes telemetric capabilities to communicate a warning signal to an external device when such arrhythmias are predicted.
The Nappholz et al. AECG monitoring device is attached to one or more leads for sensing cardiac electrical signals, which may be introduced subcutaneously into the patient's tissue using a tunneling device or a very fine blunt needle. Unfortunately, the implantation of the leads into the patient's tissue is a surgical procedure which does involve some trauma to the patient. Furthermore, movement by the patient can cause stresses upon the implanted lead, possibly resulting in dislodging or breaking of the lead, as well as irritation of the patient's tissue. Furthermore, the lead does pose some risk of infection to the patient.
The present invention provides an implantable alarm that is hermetically sealed and leadless. Because there are no leads, implantation of the alarm is minimally invasive. Furthermore, the absence of leads allows hermetic sealing of the alarm to avoid the risk of biological fluid leakage into the alarm which could render the device inoperable or cause faulty operation.
Because the alarm is implantable, its acquired physiological signals have good fidelity and consistency over time. For an alarm to chronically and constantly monitor a patient's heart condition and trigger a warning signal when medical intervention is necessary, it must be continuously reliable. Prior art monitoring devices are not sufficiently reliable for long-term monitoring. For example, electrocardiogram analysis performed using existing external or body surface AECG systems is limited by mechanical problems and poor signal quality. Electrodes attached externally to the body are a major source of signal quality problems and analysis errors because of susceptibility to interference such as muscle noise, power line interference, high frequency communication equipment interference, and baseline shift from respiration. Signal degradation in prior art AECG monitors also occurs due to contact problems, ECG waveform artifacts, and patient discomfort. Externally attached electrodes are subject to motion artifacts from positional changes and the relative displacement between the skin and the electrodes. External electrodes are impractical for long-term monitoring (longer than 24 hours).
Furthermore, external electrodes require special skin preparation and care by the patient to prevent signal corruption by dislodgement or wetting from sweating or bathing. Externally attached electrodes lack the signal fidelity required to automatically perform data analysis for automatically identifying dangerous arrhythmia conditions. Externally attached electrodes cannot chronically produce the cardiac data necessary for a reliable patient alarm. In prior art AECG recordings, physicians could pick and choose the best cardiac waveforms for visual and semi-automated analysis. A continuously responsive and chronically implanted warning device cannot select its signals for analysis. All signals must permit reliable analysis.
The implantable character of the alarm of the present invention changes the nature of a cardiac monitoring device in three ways. First, long-term and constant monitoring is practical only in an implantable system. Secondly, the absence of surface electrodes substantially increases system accuracy. Lastly, information is not altered by modification of a patient's behavior.
The present invention provides a hermetically sealed and leadless implantable alarm employing a sensor capable of measuring multiple physiological parameters. This sensor provides measurements of heart motion, respiration and patient motion. The alarm analyzes the heart motion parameter to determine a patient's heart rate. Other sensed physiological parameters may be determined by the alarm and used to derive an indication of the patient's metabolic demand. This metabolic demand indication may be further used to determine heart rate thresholds which indicate whether the patient's heart rate is normal or abnormal. Thus, an elevated heart rate will trigger the alarm when the patient is at rest but, appropriately, the same rate does not activate the alarm for an exercising patient. The alarm actives a warning signal when the heart rate is abnormal.
The sensor for the alarm of the present invention measures and analyzes impedance signals that relate to changes in impedance within the volume of tissue penetrated by the electromagnetic field generated by the sensor. These changes may be related to physiological function. The alarm determines a heart motion parameter over time, orders these parameters into a heart motion signal sequence and then analyzes this sequence to determine the patient's heart rate. The alarm may perform further analysis of the heart motion signal to determine other physiological parameters, such as stroke volume or cardiac output. For example, the alarm may process the heart motion signal to determine right ventricular stroke volume and set the patient's metabolic demand accordingly.
In one embodiment, the alarm of the present invention measures heart rate and may be programmed to activate a warning signal either when the measured heart rate exceeds a programmed maximum rate or when the measured heart rate exceeds an automatically-determined threshold heart rate derived from a physiological sensor measurement. The heart rate and physiological measurement are both acquired using the same sensor hardware.
In this manner, the present invention provides a physiological sensor, hermetically sealed within an implantable cardiac function alarm, which allows the alarm to compare the patient's heart rate to metabolic demand, determine whether the heart rate is appropriate with respect to the metabolic demand and generate a warning signal when the heart rate is inappropriate. The operation of the sensor may be altered by means of programming of the alarm from an external communicating device. These alterations in sensor operation fulfill the needs of various patients who are afflicted with different cardiac and respiratory health problems. Importantly, this sensor does not require a lead. Thus, the alarm may implanted with little trauma to the patient.
In addition to measuring heart motion, the sensor may be programmed or controlled to measure physical activity, also called patient motion, to provide an estimate of metabolic demand. One advantage of a patient motion sensor is its very rapid response time to the onset of exercise. Therefore, the patient motion sensor can track rapid increases in heart rate that would trigger a false arrhythmia alarm if a slower sensor were used to determine a patient's metabolic demand. A patient-motion sensor responds favorably to patient activities which create vibration, such as jogging, walking and stair climbing. Unfortunately, activities such as bicycling do not promote metabolic demand analysis because little vibration occurs. A disadvantage of a physical activity sensor is that it is not generally regarded as a truly physiologic sensor because it does not measure true metabolic demand and, therefore, is not affected by emotional stimuli or pyrexia. Although the lack of a truly physiological response is generally considered a disadvantage of a motion sensor, the fact that this sensor acts independently from physiologic variables may provide a better response under conditions in which patient systems or tissues are diseased. For example, a motion sensor may supply a better signal for responding to exercise than a respiration sensor will under conditions of lung disease, such as emphysema. A further disadvantage of a patient motion sensor is the difficulty of attaining a scaled response to gradations of metabolic demand. Patient motion sensors generally act in an on/off fashion, in which a sensor is unable to detect changes in patient workload. Therefore, the sensor response does not normally depend on the amount of exercise the patient is performing, but instead the measurement remains identical so long as the measured activity is above a preprogrammed level.
The sensor also may measure and analyze impedance signals which relate to a patient's respiratory function to determine metabolic demand. The respiratory parameter which correlates most closely to a patient's metabolic demand and appropriate heart rate is minute ventilation, a highly physiologic variable which reflects closely the metabolic demands of exercise. The body's increase in minute ventilation during exercise parallels its oxygen uptake but also reflects changes in cardiac output and heart rate. Minute ventilation not only correlates well with exercise, but also varies in response to stress and pyrexia. U.S. Pat. No. 4,702,253 (hereinafter called the "'253 patent"), entitled "Metabolic-Demand Pacemaker and Method of Using the Same to Determine Minute Volume" issued to T. A. Nappholz et al. on Oct. 27, 1987, discloses a rate-responsive pacemaker which senses impedance in the pleural cavity of a patient and derives respiratory minute volume from impedance. The pacemaker then employs the respiratory minute volume, a measure of the amount of air inspired by a person as a function of time, as a rate-control parameter. The greater the amount of air inspired, the greater the need for a higher pacing rate. The device described in this patent requires a nonstandard pacing lead in order to perform the minute volume measurement.
U.S. Pat. No. 4,901,725 (hereinafter called the "'725 patent"), entitled "Minute Volume Rate-Responsive Pacemaker", issued to T. A. Nappholz et al. on Feb. 20, 1990, discloses a pacemaker which performs a rate-responsive function in the manner of the '253 patent with various improvements, and, in addition, only requires standard pacing leads. To measure the intravascular impedance, the minute-volume sensor generates a low energy current pulse at 50 ms intervals between a ring electrode of the lead and the pulse generator case, then measures the voltage between the tip electrode of the lead and the pulse generator case arising from the applied current. An intravascular impedance value is determined from the measured voltage and the applied current using Ohm's law. Transthoracic impedance increases with inspiration and decreases with expiration, and its amplitude varies with the tidal volume. The impedance signal thus comprises two components, representing tidal volume and respiratory rate. Pulse generator circuitry identifies the two signals and processes them to yield minute ventilation. The minute-volume-controlled rate-responsive pacemaker employs a highly physiologic sensor. Its ability to assess the metabolic demands of the body is superior to that of a pacemaker driven by respiratory rate alone, since depth of ventilation is an important response to exercise or stress. The apparatus described in the '725 patent requires no more than standard pacing leads, although the leads cannot be unipolar leads, and programming of minute ventilation rate adaptation necessitates only a single exercise test.
Impedance sensors are also disclosed by B. M. Steinhaus et al. in U.S. Pat. No. 5,197,467, entitled "Multiple Parameter Rate-Responsive Cardiac Stimulation Apparatus", which issued Mar. 30, 1993, and in U.S. Pat. No. 5,201,808, entitled "Minute Volume Rate-Responsive Pacemaker Employing Impedance Sensing on a Unipolar Lead", which issued Apr. 13, 1993. These two patents are assigned to the assignee of the present application.